Complete your carrier-current audio system with an AM or FA4 receiver:

Transcription

1 r LAST MONTH WE WENT OVER the operating theory of a carrier-current transmitter, and then showed you how to build one. Now we will describe two receivers that can be used with that transmitter. One receiver is a simple AM unit, best suited for applications where some noise can be tolerated (such as speech), and the other one is for wideband FM use. Both receivers have an output that can be connected directly to a speaker. Figure I shows the AM-receiver schematic. It is a TRF- (Tuned Radio Frequency) type receiver, meaning that there is no signal mixing or heterodyning; the unit is tuned only once upon calibration, and then left alone. It has a sensitivity of about 1 millivolt the input for an audio output of M- Capacitor C22 couples audio signals from the power line to the PC board-it ~rzust be rated at 600-volts DC. R8 will cause F1 to blow if C22 shorts. As another safety precaution, the chassis of the unit must be groui~ded. If an older two-wire electrical system is used, the receiver must be grounded to a cold-water The signal from C22 goes to a tuned network (C1-C5 md L1 and L2) that has a 20-kHz bandwidth, which allows only the desired signal to pass through. A jumper (J) between the line (antenna) and the input network can be removed for reception of very strong signals if distortion (overload) becomes a problem. IC1 is a "gain block" IF chip, normally used for TV IF applications, but it is useful at low frequencies as well. It has AGC (Automatic Gain Control) capability and approximate- WILLIAM SHEETS and RUDOLF GRAF ly 60 db of gain. Components C8, C9, and L3, which are placed across the output of IC1, are broadly resonant around 280 khz. ClO couples RF to detector-diode Dl, which is used as an envelope detector. AGC, which keeps the receiver output relatively constant, is obtained in the following manner: The cathode of Dl is connected to the variable resistor R5. A voltage from 4 to 6 volts appears on the cathode of Dl even with no received signal. When a signal is received, the DC voltage at the anode of Dl increases. That DC voltage appears across C14 (the detector output), and is then fed through R3 and C13, which remove audio components, to the base if Q1. The voltage at the emitter of Q1 is fed to pin 5 of ICl. A more positive voltage tends to reduce the gain of IC1, which in turn reduces the signal fed to Dl and subsequently the DC voltage at pin 5. Complete your carrier-current audio system with an AM or FA4 receiver: The detector output is taken from C14, which sets the upper frequency limit at about 10 khz or so. By reducing the value of C14, a higher frequency response can be obtained; but using the FM receiver, which will be described later, is a better approach. The detector output is connected to an external iack. Audio comdonents are fed to audio-gain control R6, through C16 to IC2, an audio amplifier. C18 couples up to M-watt of audio to an external speaker. Power for the AM receiver is supplied via TI, D2, and IC3. The power supply formed by those components provides a regulated 12-volts DC across C19. FM receiver The schematic of the FM receiver is shown in Fig. 2, and it operates as follows: Input signals from the power line are coupled through C23 and R19 to the input filter network. As with the AM transmitter, C23 must be rated at 600 volts. Switch S2 is used as an attenuator. It is provided to prevent receiver overload in case it is located too close to the transmitter. Signals above about 500 khz are rejected by Cl, which reduces the tendency for the filter network to "leak" signals at frequencies far above the passband. Components C2-C7, L1-L3, and R1 and R20 form a triple-tuned bandpass filter having a passband from khz. Signals from the filter are fed to pin 4 of ICl, an MC1350P "gain block" IC, which is used as a tuned RF amplifier. C8 and C9 provide internal bypassing for the chip. R2 ; biases IC1 so that it operates at max- w n C imum gain. An amplified signal ap- D pears at pin 8 of ICI. L4 provides DC 3 bias and high RF impedance to pin 8, ; and Dl and D2 provide amplitude %

2 -4 ciz is best suited for receiving speech. 60

3 limiting of the FM signal. C10 and R3 form a decoupling network, and C11 couples the signal from IC1 to IC2, and also blocks DC. IC2, an LM.565 PLL, is used as an FM demodulator. Pins 8 and 9 are connected to an internal VCO (Voltage Controlled Oscillator), and components R9, R10, and C15 set the VCO's free running frequency. The output of the VCO appears at pin 4, and is fed right back in to pin 5, which is the input to the internal phase detector (the-test point between those two pins is used for setting the PLL's VCO frequency-280 khz in this application). The VCO signal and the input signal (from pin 2) are compared in the phase detector. The output from the phase detector is internally amplified, and then appears at pin 7. The output at pin 7 is a replica of the original modulation on the FM input TO TI FIG. >AM PARTS PLACEMENT. Follow this diagram when building the PC board for the AM receiver. signal to the receiver; the output (pin pled to Q1 and R11. Components C16, 7) is therefore the recovered audio. C29, and L5 form a low-pass filter that (The outputs from pins 6 and 7 are eliminates 280-kHz components from externally available for future inter- IC2's output. C17 and R14 couple auface purposes.) dio to the base of Q2, which, in con- The detector output at pin 7 is cou- junction with R15, R16, R17, and

4 C18, form an audio amplifier that brings the recovered audio up to around 1-volt peak-to-peak. R18 is a volume control, and L6 and C27 suppress any remaining 280-kHz components. The signal is then fed into IC3, an LM386N audio amplifier, which can deliver up to?%watt of audio, coupled via C20, to any standard 8- ohm external speaker. A power supply for the FM receiver is made up of TI, a bridge rectifier made up of D3-D6, and the 12-volt regulator, IC4. The power-supply requirements concerning ripple and noise are more stringent for the FM receiver than the AM receiver. Construction The Parts-Placement diagrams for the AM and FM receivers are shown in Figs. 3 and 4 respectively. You can build the receivers using PC boards made from the foil patterns given in PC Service, or else by using the ready-made PC boards that are available from the source mentioned in the Parts List. Complete parts kits containing all components that mount on the PC boards are also available from that same source for either receiver. Otherwise, all of the components, with the exception of the chokes and coils, are readily available from many suppliers. Of course you only need the parts for one of the receiverseither AM or FM, unless you want to build both. Following Figs. 3 and 4 as a guide, start by first installing the fixed resistors, and then the capacitors. Next, install the coils and potentiometers, and the IC's last. It's always a good idea to leave the IC's for last, as they are susceptible to static damage. Use only rosin-core solder, and be sure to carefully inspect the PC board for shorts, solder bridges, and poor solder joints before applying power. All components that are not mounted on the PC board should be mounted to the chassis or soldered onto a terminal strip, as shown in Figs. 5 and 6. In the FM receiver, as you can see from Fig. 6, voltage-regulator IC4 is actually heat-sinked by mounting it to the bottom surface of the metal cabinet; IC4 is electrically grounded to the cabinet as well. Checkout The following checks should be made before power is applied to either board: 4 FIG. 4--FM PARTS PLACEMENT. Follow this diagram when building the PC board for the FM receiver. FIG. &THE PC BOARD for the AM receiver is installed inside the cabinet as shown. n: Notice the components that are not on the PC board. AM receiver * Check all coils for DC resistance: Ll, L2, and L4 should be 48 ohms, and L3 should be 22 ohms. e IC2, pin 6 to ground: 500 ohms or more (after 10 seconds). IC2, pin 5 to ground: 10K or more. e IC1, pins 1, 2, 4-6, and 8 to ground: no shorts (should read more than 500 Make sure that Dl and D2 are correctly polarized. Set all potentiometers at halfway, apply DC power, and check for the following positive voltages (all measurements are made with respect to ground): e Across C20: 16 volts. * Across C19: 12 volts. IC2 pin 5: 6 volts. e ICl pins 1, 2, and 8: 11.8 Ql collector: 11 Q1 emitter: 6 volts (varies with R5). IC1 pin 5: 5 volts (varies with RS). FM receiver e Check all coils for DC resistance: L1-L4 and L6 should be 48 ohms, and L5 should be 22 ohms. e IC3 pin 6 to ground: more than 500 ohms (after 10 seconds). e IC3 pin 5 to ground: a?$are than continued o,,jage 94

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